This invention relates to cement compositions, and especially to cement compositions useful as mortar with refractory brickwork linings in thermal/chemical processing applications. One specific example is the high pressure autoclaves used in the acidic oxidation of refractory precious metal ores.
There are precious metal ores that are of refractory type, in which the metallic phase(s) of interest is not readily recoverable by cyanidation and other known techniques, unless the ores are first oxidized. One oxidation technique that is used is high pressure oxidation in an autoclave. Pressure oxidation methods and autoclaves are described, for example, in Thomas et al., U.S. Pat. Nos. 5,071,477 and 5,489,326. Such autoclaves are typically brickwork-lined (acid resistant refractory bricks and mortar) and the lining must withstand temperatures of 400.degree.-460.degree. F. and pressure oxidation conditions including total pressure in the range of about 400-500 psig, oxygen partial pressure of greater than about 20 psi, and agitated ore slurry contents of up to 25 or more g/L sulfuric acid.
The most common mortars now used in autoclave applications, and in the past, have included alkali silicate containing mortars and other alkali compound containing mortars. The alkali content of these mortars has limited the mortars' life because the alkali-containing phase(s) tends to react in the presence of sulfuric acid to form alkali sulfates, and other altered compounds, which are subject to significant expansion and volume change. This effect can cause or contribute to the breakdown of brickwork, because of associated stresses, cracking and strength loss that accompany the brick and mortar alteration in use.
U.S. Pat. No. 2,914,413 discloses a cold-setting acid corrosion resistant cement comprising an aqueous silica sol and solid alkali metal silicate setting agents. The cement is stated to be of reduced sodium content, due to its use of a very low sodium (less than 0.5%) source of silica for its binder, whereby problems associated with sodium content are reduced. However, potential problems associated with reaction effects, like the conversion and volume expansion of alkali metal sulfates, remain because of the incorporation of alkali metal silicate (sodium silicate) setting agents.